TNF, a pro-inflammatory cytokine produced primarily by macrophages, contributes to the defense of the host against pathogens as well as to various detrimental manifestations of inflammation through a variety of different effects on cell function (Old, 1990; Beutler and Cerami, 1989). These effects are initiated by the binding of TNF to specific, high affinity cell surface receptors (TNF-Rs), expressed on most kinds of cells (Baglioni et al, 1985; Beutler et al, 1985; Kull et al, 1985; Tsujimoto et al, 1985; Aggarwal et al, 1985; Israel et al, 1986). The receptors provide the intracellular signals for cell response to TNF (Engelmann et al, 1990a). Two molecular species of the TNF-Rs, the p55 and the p75 TNF-R, expressed differentially in different types of cells, have been identified (Engelmann et al, 1990b; Brockhaus et al, 1990).
One of the most distinct characteristics of TNF compared to other immune system mediators is its ability to elicit cell death. In many severe diseases, such as autoimmune disorders, rheumatoid arthritis, graft rejection and graft-versus-host disease, TNF seems to be a major cause for pathological tissue destruction. Blocking TNF activity in these conditions is therefore of the highest importance for the development of new therapies for these diseases.
On the other hand, in certain other situations it is advantageous to augment the function of TNF, e.g., in situations where its anti-tumor or antibacterial activities are beneficial.
The main mediator for the cytotoxic effect of TNF on fibroblastoid and epithelial cells is the p55 TNF-R, which also is the prevalent TNF-R type on these cell lines. Blocking this receptor species abolishes the cytocidal effect of TNF, while inducing aggregation of the receptor molecules can mimic the cytocidal effect of TNF.
The soluble form of this receptor, as well as the soluble form of the other (p75) TNF-R, have been shown to have inhibitory effects on TNF function. Evidence was presented that these soluble forms are derived proteolytically from the cell surface forms, (Nophar et al, 1990; Porteu and Nathan, 1990; Porteu et al, 1991).
Both the signaling by the receptors and the cleavage of the receptors appear to reflect interactions between the receptor and some cellular effector proteins; the cleavage—by interacting with regulatable proteases, and the signaling—by interacting with proteins having some signaling activity.